Faced with myriad external insults, like temperature changes and osmolarity imbalances, cells must adjust their biochemical activities to meet ever-shifting demands. To counteract environmental challenges, or stresses, cells have evolved a collection of stress response pathways that work as corrective feedback loops to restore homeostasis when the cell is thrown out of equilibrium. Stress responses are ancient and the core pathways are conserved in all eukaryotes. Defects in these pathways - failures to restore homeostasis - can have deleterious effects, as in disease states like diabetes. Moreover, pathogens and cancers can selectively modulate and exploit stress response pathways to harness their cytoprotective functions. While decades of genetics, biochemistry and expression profiling have identified the pathways, worked out the basic activation mechanisms and revealed the target genes our current understanding of stress response pathways lacks both depth and breadth. It lacks depth in that we do not know the mechanisms that control the pathways in real-time to ensure sufficient activation upon stress and efficient deactivation once homeostasis is restored. Our understanding lacks breadth in that the pathways have generally been studied independently, neglecting potential interconnections. A quantitative and mechanistic understanding of how these pathways are regulated to restore homeostasis and knowledge of how the different stress responses operate as an interconnected network are prerequisites to effectively modulating these pathways for therapeutic purposes. In this context, I propose three specific aims to increase the depth of our understanding of the quantitative regulatory mechanisms that control stress response pathways and the breadth of our understanding of the interconnections between these responses. In the first two aims I will focus on the heat shock response, the elemental and and prototypical stress response, to reveal how phosphorylation and chaperone protein binding dynamics quantitatively regulate the activity of the transcription factor, Hsf1. In the third aim, I will focus on the interconnections between stres response pathways by building a panel of stress reporter strains that will allow simultaneous measurement of all stress responses following any genetic or environmental perturbation. The proposed research is significant because it will provide depth and breadth to our understanding of stress responses. Such understanding is a prerequisite to effectively harnessing these vital pathways for therapeutic benefit. Finally, I expect that the mechanistic systems biology approach described here will serve as a model for the quantitative investigation of pathways and networks in increasingly complex systems.